vidioc-g-fmt.rst 6.0 KB
Newer Older
1 2
.. -*- coding: utf-8; mode: rst -*-

3
.. _VIDIOC_G_FMT:
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43

************************************************
ioctl VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT
************************************************

*man VIDIOC_G_FMT(2)*

VIDIOC_S_FMT
VIDIOC_TRY_FMT
Get or set the data format, try a format


Synopsis
========

.. c:function:: int ioctl( int fd, int request, struct v4l2_format *argp )

Arguments
=========

``fd``
    File descriptor returned by :ref:`open() <func-open>`.

``request``
    VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT

``argp``


Description
===========

These ioctls are used to negotiate the format of data (typically image
format) exchanged between driver and application.

To query the current parameters applications set the ``type`` field of a
struct :c:type:`struct v4l2_format` to the respective buffer (stream)
type. For example video capture devices use
``V4L2_BUF_TYPE_VIDEO_CAPTURE`` or
``V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE``. When the application calls the
44
:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` ioctl with a pointer to this structure the driver fills
45 46 47 48 49 50 51 52 53 54 55 56
the respective member of the ``fmt`` union. In case of video capture
devices that is either the struct
:ref:`v4l2_pix_format <v4l2-pix-format>` ``pix`` or the struct
:ref:`v4l2_pix_format_mplane <v4l2-pix-format-mplane>` ``pix_mp``
member. When the requested buffer type is not supported drivers return
an EINVAL error code.

To change the current format parameters applications initialize the
``type`` field and all fields of the respective ``fmt`` union member.
For details see the documentation of the various devices types in
:ref:`devices`. Good practice is to query the current parameters
first, and to modify only those parameters not suitable for the
57
application. When the application calls the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl with
58 59 60 61 62 63
a pointer to a :c:type:`struct v4l2_format` structure the driver
checks and adjusts the parameters against hardware abilities. Drivers
should not return an error code unless the ``type`` field is invalid,
this is a mechanism to fathom device capabilities and to approach
parameters acceptable for both the application and driver. On success
the driver may program the hardware, allocate resources and generally
64
prepare for data exchange. Finally the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl returns
65
the current format parameters as :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` does. Very simple,
66 67
inflexible devices may even ignore all input and always return the
default parameters. However all V4L2 devices exchanging data with the
68
application must implement the :ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` and :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`
69
ioctl. When the requested buffer type is not supported drivers return an
70
EINVAL error code on a :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` attempt. When I/O is already in
71 72 73
progress or the resource is not available for other reasons drivers
return the EBUSY error code.

74
The :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctl is equivalent to :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` with one
75 76 77 78 79 80
exception: it does not change driver state. It can also be called at any
time, never returning EBUSY. This function is provided to negotiate
parameters, to learn about hardware limitations, without disabling I/O
or possibly time consuming hardware preparations. Although strongly
recommended drivers are not required to implement this ioctl.

81 82
The format as returned by :ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` must be identical to what
:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` returns for the same input or output.
83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198


.. _v4l2-format:

.. flat-table:: struct v4l2_format
    :header-rows:  0
    :stub-columns: 0


    -  .. row 1

       -  __u32

       -  ``type``

       -  
       -  Type of the data stream, see :ref:`v4l2-buf-type`.

    -  .. row 2

       -  union

       -  ``fmt``

    -  .. row 3

       -  
       -  struct :ref:`v4l2_pix_format <v4l2-pix-format>`

       -  ``pix``

       -  Definition of an image format, see :ref:`pixfmt`, used by video
          capture and output devices.

    -  .. row 4

       -  
       -  struct :ref:`v4l2_pix_format_mplane <v4l2-pix-format-mplane>`

       -  ``pix_mp``

       -  Definition of an image format, see :ref:`pixfmt`, used by video
          capture and output devices that support the
          :ref:`multi-planar version of the API <planar-apis>`.

    -  .. row 5

       -  
       -  struct :ref:`v4l2_window <v4l2-window>`

       -  ``win``

       -  Definition of an overlaid image, see :ref:`overlay`, used by
          video overlay devices.

    -  .. row 6

       -  
       -  struct :ref:`v4l2_vbi_format <v4l2-vbi-format>`

       -  ``vbi``

       -  Raw VBI capture or output parameters. This is discussed in more
          detail in :ref:`raw-vbi`. Used by raw VBI capture and output
          devices.

    -  .. row 7

       -  
       -  struct :ref:`v4l2_sliced_vbi_format <v4l2-sliced-vbi-format>`

       -  ``sliced``

       -  Sliced VBI capture or output parameters. See :ref:`sliced` for
          details. Used by sliced VBI capture and output devices.

    -  .. row 8

       -  
       -  struct :ref:`v4l2_sdr_format <v4l2-sdr-format>`

       -  ``sdr``

       -  Definition of a data format, see :ref:`pixfmt`, used by SDR
          capture and output devices.

    -  .. row 9

       -  
       -  __u8

       -  ``raw_data``\ [200]

       -  Place holder for future extensions.



Return Value
============

On success 0 is returned, on error -1 and the ``errno`` variable is set
appropriately. The generic error codes are described at the
:ref:`Generic Error Codes <gen-errors>` chapter.

EINVAL
    The struct :ref:`v4l2_format <v4l2-format>` ``type`` field is
    invalid or the requested buffer type not supported.


.. ------------------------------------------------------------------------------
.. This file was automatically converted from DocBook-XML with the dbxml
.. library (https://github.com/return42/sphkerneldoc). The origin XML comes
.. from the linux kernel, refer to:
..
.. * https://github.com/torvalds/linux/tree/master/Documentation/DocBook
.. ------------------------------------------------------------------------------